Michael
I Love YTtalk
Givaways will get people to subscribe to you no matter if you mention it of not, but they soon unsubscribe when they don't win or see that you are not giving away stuff left and right. People love free stuff.
Thats the biggest issue with them, YouTubes rules are quite finicky as they are and even if you do gain a lot from them you do get many who unsubscribe once they have had their fill or not got theirs. It is the same reason I wont do competitions for real money here regularly, I dont mind once in a while but if we started doing it all the time people would come for the prizes then leave.
I have known a channel to do giveaways every few weeks however back in 2010-2011 and they have a huge subscriber base now and I believe they giveaways must have contributed to it.